I’ve added a new entry to the FAQ regarding negative amortizations. In the process, I also noticed that some of the summary information produced by the calculator and final schedule row are not completely consistent in the NegAm scenario. I will need to fix these. It is apparent that I missed some sort of symmetry in my calculations here (I probably took a shortcut to minimize computer time, which is just so much gagging at gnats these days).
This is interesting to me because I’ve recently been reading some of the Richard Feynman Lectures on Physics, and he talks a bit about mathematical symmetries. We have such an expectation of symmetry that lack of it in certain contexts may indicate a flaw in our understanding, or sometimes it may lead to the discovery of even more interesting properties. (Amortization is not so glamorous as a fundamental law of the universe, mind you; but I now recognize that had I paid closer attention to a mathematical symmetry in this case, I might have noticed my error sooner.)